1. Field of the Invention
The present invention relates to x-radiators, and in particular to an x-radiator having means for preventing migration of the focal spot of the electron stream on a rotating anode.
2. Description of the Prior Art
An x-radiator is described in British Pat. No. 365 432, wherein a thermionic cathode generates an electron stream which is incident on an anode disk or anode dish, the electron stream being deflected by an external magnetic field so as to be incident on an off-center focal spot on the anode. The x-radiator described in British Pat. No. 365 432 was developed at the time that the principle of rotating anodes was first introduced into x-ray technology, and the entire tube, with the cathode and anode rigidly mounted therein, was intended to be rotated about the longitudinal axis of the arrangement. The external magnetic field was stationary, and the entire tube arrangement was displaced with respect to the magnetic field so as to result in the desired deflection of the electrons. Due to substantial mechanical difficulties, this arrangement did not prove to be practical.
In rotating anode x-ray tubes which were subsequently developed, only the anode itself was made to rotate. In such tubes, the cathode is rigidly mounted in the bulb of the tube at a radial distance from the longitudinal axis of the arrangement, which is coaxial with the center of the anode. The spacing corresponds to the radius of the path of the focal spot. It is thus not necessary in conventional tubes of this type to locate the electron stream by a magnetic field. The electron stream may nonetheless be influenced by extraneous magnetic fields, such as the earth's magnetic field, and the path of the stream is correspondingly deflected. This causes a slight change in the position of the focal spot on the anode Since the position of the focal spot determines the position of the emitted x-ray beam, such migration is undesirable, particularly in computer tomography, wherein a radiation source having a specific position is of importance.
Migration of the focal spot in convention x-ray tubes may occur for other reasons such as thermal expansion of components of the tube occurring during manufactured or during operation, which causes geometrical changes in the relative positions of the electrodes. The rotating anode can also cause migration of the focal spot due to vibrations induced by the mechanical rotation